3,6-Dichloropicolinic acid, a highly active plant growth regulator, can be prepared by any of the methods disclosed in U.S. Pat. Nos. 3,317,549; 4,087,431 or 4,217,185. In the method of the U.S. Pat. No. 3,317,549 patent, the acid is obtained, as such, by hydrolysis of 3,6-dichloro-2-trichloromethyl pyridine with an aqueous solution of a mineral acid. In the methods of both of the other two patents, the 3,6-dichloropicolinic acid is obtained as a basic, aqueous solution of a salt--such as sodium 3,6-dichloropicolinate, for example--from which it is liberated ("sprung") as a solid, by acidification with a mineral acid.
The picolinate solution obtained in the method of the U.S. Pat. No. 4,087,431 patent is formed by refluxing 3,5,6-trichloro-4-hydrazino-picolinic acid with a solution of 2 to 3 molecular proportions of a base, such as sodium carbonate, in an aqueous medium, such as water or ethanol/water. The reaction mixture is cooled and acidified (any water-miscible solvents being removed). The resulting free 3,6-dichloropicolinic acid is recovered by filtration or is taken up in a water-immiscible solvent, such as dichloromethane, and then recovered by conventional procedures from the resulting solution. In commercial practice of the U.S. Pat. No. 4,087,431 process, organic solvents are not employed and about 80% of the picolinate formed is recovered as the free acid by heating the picolinate solution, acidifying it, cooling it slowly, filtering out the precipitated crystals and disposing of the filtrate (in an environmentally acceptable manner).
The picolinate obtained in the method of the U.S. Pat. No. 4,217,185 patent is formed by electrolytic reduction of tetrachloropicolinic acid in basic, aqueous solution, at an activated silver cathode. The free dichloro acid has been recovered in pilot plant operations, up to about 98% pure and in overall yields of up to 99% of theoretical, by extraction of the acidified picolinate solution with dichloromethane and evaporation of the extract. Such yields and purities are generally not economically reproducible on a commercial scale but at least give promise of bettering the results experienced in the commercial practice of the U.S. Pat. No. 4,087,431 process.